This invention relates to an improvement of a magnetic recording and playback apparatus of perpendicular recording type which is used to achieve a high density magnetic recording on a magnetic record medium.
A conventional magnetic recording process which utilizes a ring-shaped head, as may be used in a tape recorder, is based on the recording or playback of magnetization vectors which are oriented lengthwise of a record medium. When the process is used to increase the recording density, not only the strength of magnetization decreases in a range of shorter wavelengths due to the demagnetization effect within the record medium, but a rotation of magnetization vectors occurs in the cross section of the medium, thus substantially reducing a reproduced signal output to defeat the very purpose of achieving a high density recording. To solve this problem, there has been proposed a perpendicular magnetic recording process (see Japanese laid-open patent applications No. 134,706/1977 and No. 34,205/1979) in which the demagnetization effect is reduced to substantially zero.
One of such techniques utilizes a main pole formed by a thin magnetic strip of a high permeability which is coated with a soft material, and an auxiliary pole on which a record/playback coil is disposed, both of which are disposed in opposing relationship with each other with a record medium placed therebetween. In this manner, a magnetic flux from the auxiliary pole is concentrated onto the recording axis of the main pole to achieve a local inversion of magnetization in the record medium, thus recording information. However, the technique suffers from the following disadvantages:
1. Because the main pole and the auxiliary pole are disposed in opposing relationship with each other with the record medium placed therebetween, a difficulty is experienced in positioning the poles relative to each other, particularly when a multihead device is contemplated.
2. A magnetic circuit formed by the combination of the main and auxiliary pole represents an open magnetic path, whereby magnetic lines of force spread largely into the open space to increase the reluctance of the magnetic circuit and to degrade the energy efficiency. Thus, it is necessary to pass a current of an increased magnitude through the coil on the auxiliary pole when recording.
3. When reproducing recorded information, the magnitude of an output depends on the strength of magnetization vectors, which represent a signal source in the record medium, and is inversely proportional to the reluctance of the magnetic circuit, so that the signal-to-noise ratio is very poor during the playback.
An apparatus according to another proposed technique utilizes a recording yoke disposed in opposing relationship with a recording surface of a record medium and having a surface of a relatively small area which is disposed opposite to the record medium. In addition, an auxiliary yoke is disposed in surrounding relationship with the recording yoke except for its portion located opposite to the recording surface, and having a surface of a relatively large area which is disposed opposite to the record medium. A coil is disposed on the auxiliary yoke. When recording, magnetic lines of force form a closed magnetic path including the auxiliary yoke, the record medium and the recording yoke. Flux from the auxiliary yoke is concentrated into the recording yoke, which is effective to write information into the record medium.
While flux concentration into the recording yoke is achieved when recording, the technique suffers from the fact that a reduced area (S) and an increased length (l) of the recording yoke result in an increased magnitude of reluctance (Rm), as expressed by the following relationship: EQU Rm.alpha.l/S
Because no magnetic shield is provided around the recording yoke, a leakage of flux from the recording yoke may make the recording and playback uncertain.